Method and apparatus for setting a clamping load

ABSTRACT

The clamping preload in a molding machine or the like is set by adjusting the length of tie bars which support two die plates for opening and closing movement and a thrust plate. A clamping mechanism of given stroke is positioned between the thrust plate and one of the disc plates, and operates to close the die plates at one limit of the clamping stroke. With the clamping mechanism set at its limit and the tie bars set to establish a slight spacing between the die plates, the length of the tie bars is adjusted by means of a drive motor until the die plates are in contact without substantial preload. The clamping mechanism then opens the die plates and the drive motor is further energized to adjust the length of the tie bars and bring the die plates closer to one another by an amount corresponding to a preselected preload. The adjustment is measured from a high ratio mechanical drive connecting the drive motor to the adjustable tie bars, and the measured adjustment is compared with a desired adjustment corresponding to the preselected preload to deenergize the drive motor when the desired adjustment is reached.

This is a division, of application Ser. No. 080,180 filed Oct. 1, 1979.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for setting thepreload between die plates such as found in molding, stamping andpressing machines. More particularly, the invention is concerned withsetting the preload between two die plates that are operated by aclamping mechanism of given stroke.

In injection molding machines of the prior art, a pair of die plates arereciprocated relative to one another to open and close molds defined orcarried by the plates. A toggle linkage is customarily employed todevelop the high clamping forces holding the die plates together duringthe injection process. The linkage reacts the clamping forces through athrust plate and a set of adjustable tie bars that extend between thethrust plate and the die plates. The clamping load is sometimes referredto as the preload because it is developed by the linkage as soon as themolds are closed.

It is desirable to set the "die height" or preload on the die plateswhen different molds or other changes are made in the molding machine.The preload is directly related to the amount by which the tie bars arestrained when the toggle linkage places the dies in a lock-up condition.Thus, setting the length of the tie bars which are strained between thedie plates and thurst plate sets the preload. The effective tie barlength can be set by means of a motor which rotates nuts on the ends ofthe tie bars and moves the thrust plate relative to the die plates. U.S.Pat. No. 2,498,264 to Goldhard discloses a set of motor-driven nuts ofthis type and a control mechanism for deenergizing the motor when aparticular setting is obtained.

The strain of the tie bars associated with the preload is a smallquantity compared to the relative displacement of the die plates in amolding operation; therefore, the mechanism for adjusting that strainmust be accurate. In the Goldhard patent referenced above, a limitswitch that can be adjusted in position for various preloads is actuatedby one of the die plates as the tie bar length for a particularoperation is reached. Since the adjustment in length must be set tothousandths of an inch, the limit switch actuated by the die plate,thereby measuring absolute position of the plate, makes accuracy andrepeatability difficult to achieve.

It is accordingly a general object of the present invention to providemethod and apparatus for setting the preload or die height between twoclamped members in molding machines and the like.

SUMMARY OF THE INVENTION

The present invention resides in a machine for molding, stamping,pressing or the like in which a first die plate is reciprocated betweenopen and closed positions relative to a second die plate in a cyclicmachine operation. A clamping mechanism for opening and closing the dieplates is connected between the first die plate and a thrust plate tiedto the second die plate for reacting the clamping load between theplates in a closed position. Drive means used for setting die heightmoves the thrust plate and the second die plate relative to one anotherand thereby sets a corresponding clamping load between the plates in theclosed position.

Apparatus for establishing a predetermined clamping load between the dieplates comprises sensing means operatively connected with the drivemeans for measuring the movement of the thrust plate and second dieplate from one relative position to another by the drive means. Forexample, the drive means may include a high ratio mechanical drivehaving a large input displacement and small output displacement thatfacilitates relative movement of the thrust plate and die plate. Asensor associated with the large displacement input accurately measuresdisplacements of the order needed to set the preload.

The invention also relates to the method by which the preload is set ina machine having a clamping mechanism of given stroke. An adjustablelinkage, such as a tie bar, is provided between the clamping mechanismand one of the die plates to adjust the stress in the linkage and thecorresponding preload between the plates in the closed position at onelimit of the clamping stroke. With the clamping mechanism at the limit,the linkage between the clamping mechanism and one of the die plates isadjusted to a first length which brings the two die plates into a knownposition relative to one another, for example, in contact without anysignificant preload. Thereafter, the length of the adjustable linkage ischanged by a selected amount from the first to a second length whichpreloads the die plates correspondingly at the limit of the clampingstroke. The adjustment of the linkage is performed while the clampingmechanism is holding the die plates in spaced relationship so that theadjustment is not opposed by the loads established by the adjustment.

The setting of the linkage for a particular preload can be accomplishedautomatically by providing motor means for changing the length of theadjustable linkage and a sensor for measuring the change in length asthe motor is driven. By comparing the change as it occurs with a desiredchange and corresponding preload, the motor may be automaticallydeenergized at the proper adjustment.

A principal advantage of the present invention is that the preloadsetting is obtained more accurately by measuring incremental changes inthe length of the adjustable linkage from a known positionalrelationship of the die plates. Small relative movements are measuredrather than an absolute position of the thrust plate. The method andapparatus allow changes in the preload to be rapidly set in the machine,and the preload for particular molds, clamps or other devices may berecorded and reestablished at any time without difficulty.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an injection molding machine in whichthe present invention is employed, and shows the machine with the dieplates closed.

FIG. 2 illustrates the injection molding machine in FIG. 1 with the dieplates in an open position.

FIG. 3 illustrates the molding machine in FIG. 1 with the length of thetie bars set to establish clearance between the die plates when theclamping mechanism is at one limit of its stroke.

FIG. 4 illustrates the molding machine of FIG. 1 during the step ofadjusting the length of the tie bars.

FIG. 5 is a perspective view showing the backside of the thrust plateand the preload adjusting apparatus in the molding machine of FIG. 1.

FIG. 6 is a schematic drawing of the control circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an injection molding machine, generally designated10, incorporating and utilizing the concepts of the present invention.The machine 10 has a stationary die plate 12, a movable die plate 14 anda thrust plate 16, all of which are mounted in parallel relationshipwith one another on four tie bars 20, 22, 24, 26 as shown in FIGS. 1 and5. The die plate 12 includes a mold cavity 30 which is supplied withmelted plastic or other settable material from a heated injectioncylinder 32. The injection cylinder receives the plastic material inpowdered or pellet form through the hopper 38 and a charge of thematerial is loaded into the cylinder by means of a rotary drive motor 36at the end of the cylinder opposite from the mold 30. The die plate 14includes a parison pin 34 (FIG. 2) which defines the shape of an objectto be formed from the settable material in conjunction with the cavity30.

During a molding operation the die plates are moved to a closed positionsuch as illustrated in FIG. 1 and are clamped together with a preloadsufficient to withstand the injection pressures and prevent separationor movement at the interface of the die plates. Flashing, that is, athin film of plastic emanating from the molded part at the seam betweenseparated or moved die plates, is prevented by the preload which may beon the order of several hundred tons for large machines.

In the illustrated molding machine 10, the clamping force is produced bya clamping mechanism including a hydraulic cylinder 40 mounted in thethrust plate 16 and a toggle mechanism actuated by the cylinder 40 andincluding a cross-head 42 and upper and lower toggle linkages 44. Whenthe cylinder 40 has been extended as shown in FIG. 1, the togglelinkages 44 in the toggle position produce the preload between the dieplates 12 and 14, and that load is reacted from the die plate 14 and thetoggle linkages through the thrust plate 16 to the tie bars 20-26 andthe die plate 12. The preload which is reacted through the tie barselongates or strains the bars by an amount which is directlyproportional to the load. The fixed nuts 48 in FIG. 1 on the one end ofthe tie bars and the adjusting nuts 50, 52, 54 and 56 in FIGS. 1 and 5on the opposite ends of the tie bars establish the effective length ofthe tie rods supporting and strained by the preload. By increasing ordecreasing the length of the tie bars, the preload is set since thetoggle linkages 44 in conjunction with the cylinder 40 have a given orfixed stroke and always clamp the die plates in the toggle position atthe one limit of the stroke.

To this end, the adjusting nuts 50-56 shown in greater detail in FIG. 5are rotatably driven and adjust the length of the tie bars 20-26respectively. (The tie bar 20 is intentionally omitted in the nut 50 inorder to illustrate the threading therein). The nuts 50-56 are drivensimultaneously by means of a drive motor 58 and a chain-and-sprocketsystem including sprockets 60, 62, 64 and 66 mounted respectively on theadjusting nuts 50, 52, 54, 56 and a common chain 68. The adjusting nuts50-56 together with the sprockets 60-66, the drive chain 68 and thedrive sprocket 70 on the shaft of the motor 58 constitute a high ratiomechanical drive or reduction drive from the motor 58 primarily due tothe threading between the nuts and the tie bars and the relative sizesof the sprockets 60-66 and the drive sprocket 70. As a result arelatively large rotational displacement, that is, several rotations ofthe motor drive shaft, produce a relatively small axial displacement ofthe adjusting nuts and the thrust plate 16 on the tie bars. Of course,the effective length of the tie bars changes correspondingly, and anaccurate setting of the preload is brought about as described in greaterdetail below.

To establish a predetermined preload between the die plates 12 and 14,the adjusting nuts 50-56 are rotated by the drive motor 58 until the dieplates 12 and 14 are clearly separated when the toggle linkages areextended in the toggle position as shown in FIG. 3. Preferably thehydraulic cylinder 40 is first actuated to draw the die plate 14 awayfrom the die plate 12 as shown in FIG. 2 so that a relatively low powermotor 58 can rotate the adjusting nuts in the counter-clockwisedirection and backoff the adjusting nuts without a clamping load. Itwill be understood that since the sprockets, drive chain and nutsconstitute a high ratio drive, and adjustment of the die plates can becarried out at a no load condition, the motor 58 only needs to produceenough power to overcome friction and make the desired adjustment.

After the die plates have been separated, the drive motor 58 isenergized to rotate the adjusting nuts 50-56 and move the thrust plate16, the die plate 14, and extended toggle linkages 44 toward thestationary die plate 12. When the die plate 14 makes contact with theplate 12 without any substantial preload, that is, when the die platesare "kissing," the drive motor 58 is deenergized. Preferably the motor58 is allowed to drive the plate 14 against the plate 12 until the motorstalls and then the motor is deenergized. In this fashion a uniformreference point is established for subsequent steps in setting thepreload.

It will be understood that further adjustment of the drive nuts 50-56beyond the positions in which the die plates 12 and 14 are brought into"kissing" relationship, results in extension or strain of the tie rodswhen the die plates are closed, and locked up, and produces acorresponding preload of the die plates against one another. Suchadjustment of the tie rods can be measured with high accuracy by meansof a displacement sensor connected with the input end of the high ratiodrive mechanism between the motor 58 and the tie rods 20-26. The sensordoes not need to be a high accuracy displacement sensor because thedisplacement of the chain 68 by the motor 58 is relatively largecompared to the displacement of the adjusting nuts and thrust plate 16on the tie rods, the displacements being related by the ratio of themechanical drive.

To this end a displacement sensor comprised by a toothed plate 80 and aproximity switch 84 is driven from the chain 68 by means of a sprocket82. The proximity switch may be an optical or magnetic sensor whichdetects each tooth of the plate and produces a corresponding outputpulse as the plate is rotated by the sprocket in front of the switch.Each pulse represents an incremental displacement of the chain 68 and aninfinitesimal change in the effective length of the tie bars 20-26. Thedifference in diameter of the toothed plate 80 and the sprocket 82improves the resolution of chain displacement sensed by the switch 84,and the proportional displacement and adjustment of the nuts.

The proximity switch 84 is only used to measure the nut adjustment fromthe reference position established by bringing the die plates intokissing relationship and the final position that establishes thepreload. Since the drive motor 58 is not capable of driving theadjusting nuts 50-56 while the die plates 12 and 14 are in kissingrelationship, it is necessary to open the die plates by means of theclamping mechanism as shown in FIG. 4. The toggle linkages 44 are pulledfrom their toggle position and then the drive motor 58 is energized.

The movement of the adjusting nuts 50-56 and the thrust plate 16relative to the tie bars 20-26 is measured by counting the pulses fromthe proximity switch 84. With a direct relationship existing between thenumber of pulses and nut displacement, and since the displacement can berelated to the preload or lockup load by virtue of the spring constantof the system, the output pulses of the switch 84 are a measure of thepreload that will exist between the die plates 12 and 14 when the togglelinkages 44 are subsequently placed in their toggle position. Thus, atable of values representing the number of pulses generated by theproximity switch can be established for given preloads of the moldingmachine 10.

To use the preload table of values it will be, of course, necessary todetermine how much preload is required for a given set of molds. Whenthe smallest preload that properly produces articles from a particularset of molds has been determined, the machine can be set and reset timeand time again at that preload by carrying out the adjusting processdescribed above and matching the pulse count from the proximity switchwith the table value. The preloads are accurate since the pulse countrepresents the large displacement input of the high ratio drive, and theaccurate setting is made without a great deal of difficulty from theoutput signals of the switch.

The capability of setting the preload is highly desirable since manydifferent molds are used in injection molding machines, and each set ofmolds requires a unique preload to properly form the molded articles. Analternative is to preload the die plates at the maximum tonnage of themachine all of the time; however, such a procedure needlessly reducesthe life of the mold, and may also reduce the life of the machine byrequiring the machine to cycle at higher stress levels which acceleratewear and failure due to fatigue. Consequently, the ability to adjust thepreload accurately and to repeatedly set a particular preload in themachine offers greater flexibility of the machine in the hands of thecustomer and a longer useful life of the molds and machine.

FIG. 6 is a schematic diagram of machine controls that allow the preloadof the molding machine 10 to be set automatically after a desiredpreload has been programmed in the machine. A sensor 100 is provided tomeasure the relative movement of the adjusting nuts 50-56 along the tiebars 20-26. For example, the toothed plate 80, sprocket 82 and theproximity switch 84 in FIG. 5 provide an accurate measure of the nutdisplacement and hence, are quite suitable for the sensor 100 in FIG. 6.As a pulse generator, the proximity switch 84 produces a pulsed outputsignal proportional to the displacement, and that signal is transmittedto a pulse counter 102 for conversion to a digital or analog signal ofcorresponding value. The pulse counter is only energized to record thepulse count during the interval in which the drive motor 58 is energizedto move the thrust plate 16 from the reference position that brings thedie plates 12 and 14 into kissing relationship and the adjusted positionwhich produces the desired preload between the die plates. In thismanner the pulse count represents the relative movement of the thrustplate from the reference position rather than an absolute position.

The pulse count is then compared with a count stored in a preset memory104 that has been programmed with a pulse count corresponding to adesired preload. Comparison of the programmed count in the memory 104and the actual count in the counter 102 is made by the comparatorcircuit 106 to determine when the counts are the same, and when acoincidence of the counts is detected, the comparator deenergizes thedrive motor 58 automatically through the motor control 108. When themotor is deenergized, rotation of the nuts 50-56 stops and the thrustplate 16 is then in a position along the tie bars 20-26 that producesthe predetermined preload when the toggle linkages 44 of the clampingmechanism close the die plates and hold the plates in a preloaded stateat the toggle position.

Accordingly, an apparatus and method have been disclosed for preloadingthe die plates of a molding machine in a simple, high accuracy andspeedy process. Accuracy is achieved by utilizing a sensor whichmeasures only relative movement of the adjusting linkage, that is, tiebars which connect the clamping mechanism with the die plates. Thesensor also provides an accurate measure for preloading the plates sinceit operates upon the large displacement input of a high ratio mechanicaldrive adjusting that linkage. The speed with which an operator of themachine can set a particular preload is improved by the simplicity ofthe controls and their use in the process.

While the present invention has been described in a preferredembodiment, it should be understood that numerous modifications andsubstitutions can be had without departing from the spirit of theinvention. For example, although the invention has been disclosed in amolding machine, its utility is not limited in this respect but hasapplicability to stamping, cutting and other types of pressing machinesin which a clamp mechanism is required to load the die plates. Togglelinkages 44 are typically used in machines of this type, but other typesof clamping mechanisms having a given stroke may also be used. Theproximity switch serving as a pulse generator for measuring theincremental displacement of the adjusting nuts and thrust plate from oneposition to another with high accuracy is but one of a plurality ofdisplacement transducers that can serve the measuring function.Accordingly, the present invention has been described in a preferredembodiment by way of illustration rather than limitation.

I claim:
 1. A method of setting the preload between two die plates thatare moved between opened and closed positions by means of a clampingmechanism of given stroke comprising:providing an adjustable linkagebetween the clamping mechanism of given stroke and one of the die platesopened and closed by the clamping mechanism to adjust the stress in thelinkage and the corresponding preload between the plates in the closedposition at one limit of the clamping mechanism stroke; adjusting thelinkage between the clamping mechanism and said one of the die plates toa first length which separates the two die plates when the clampingmechanism is at said one limit of the stroke; with the clampingmechanism at said one limit of the stroke, adjusting the linkage betweenthe clamping mechanism and said one of the die plates from the firstlength to a second length which brings the two die plates into a knownposition relative to one another; and then moving the clamping mechanismfrom said one limit of the given stroke toward the other limit toestablish a separation between the die plates with the adjustablelinkage at the second length; and then changing the length of theadjustable linkage by a measured amount from the second length to athird length which preloads the die plates in the closed position by acorresponding amount when the clamping mechanism is subsequentlyreturned to said one limit of the clamping mechanism stroke.
 2. A methodof setting the preload as defined in claim 1 wherein the step ofadjusting the linkage to a second length which brings the two die platesinto a known position comprises bringing the two die plates togetherwithout any significant preload.
 3. A method of setting the preloadbetween two die plates as defined in claim 2 wherein:the step ofproviding an adjustable linkage includes providing a motor and drivemeans of sufficient size and strength for adjusting the length of thelinkage only when the die plates are separated and not under load; andthe step of adjusting the linkage from the first to a second lengthcomprises energizing the motor and drive means to adjust the length ofthe linkage and move the die plates together, and allowing the motor tobring the plates together and stall without significant preload.
 4. Amethod of setting the preload between two die plates as defined in claim1 wherein the step of changing the length of the adjustable linkageincludes measuring the change in length of the linkage from the secondto the third length and producing a signal indicative of the measuredchange to confirm a change by the measured amount.
 5. A method ofsetting the preload as defined in claim 4 wherein:the step of providingincludes providing a motor for adjusting the length of the linkage; thestep of changing the length of the adjustable linkage comprisesenergizing the motor, comparing the produced signal for the change inlength of the linkage with a predetermined value for the signal at apredetermined change in length and corresponding preload; andautomatically deenergizing the motor when the produced signal and thepredetermined value are the same.
 6. A method of setting the preloadbetween two die plates as defined in claim 1 wherein the step ofproviding an adjustable linkage between the clamping mechanism and oneof the die plates comprises providing a thrust plate and mounting thethrust plate and the two die plates on adjustable length tie bars withthe thrust plate and said one of the die plates at opposite sides of theother of the die plates, and the clamping mechanism connected betweenthe other of the die plates and the thrust plate.
 7. A method of settingthe preload between two die plates as defined in claim 1 or 6 whereinthe clamping mechanism comprises a toggle linkage connected to move thedie plates toward the closed position at the one limit of the strokedefined by the toggle position of the linkage.